Histological Insights into the Neomorphic Preparietal of Dicynodont Therapsids

Neomorphic skull bones are rare among tetrapods, appearing only a handful of times. One notable example is the preparietal, which is thought to have evolved three times in therapids within dicynodonts, biarmosuchians, and gorgonopsians. Here, we will be histologically describing the preparietal in dicynodonts, a non-mammalian synapsid from about 260 million years ago. We’ll be looking at the preparietal in specimens of two dicynodonts: Diictodon feliceps and an indeterminate species of Lystrosaurus. For both, we made coronal thin sections through the skull roof in the region containing the frontal, the preparietal, and the parietal foramen. It has been noted that the preparietal shape varies substantially in dorsal view, and we found a similar pattern in thin-section. In the Diictodon specimen, which likely represents a mature individual, the anterior thin-sections show that the preparietal forms fan-like prongs that embed themselves entirely within the frontal bone. Additionally, there is evidence of an anterior midline suture. In the Lystrosaurus specimen, which shows evidence of being a juvenile, there is a well-defined midline suture in the posterior section of the preparietal, but the prongs and midline suture were not present anteriorly. In both taxa, there is an interdigitated suture that is formed between the anterior portion of the preparietal and the underlying frontal and parietal bones. We also found highly vascularized fibrolamellar bone in the posterior sections of the preparietal in both taxa, suggesting rapid growth in the posteroventral direction. Because our data shows these two features present in both taxa, we believe that these are characteristics of the dicynodont preparietal. More histological work will need to be done on gorgonopsians and biarmosuchians to determine if the histological features characterizing dicynodonts are also found in the other groups of therapsids that evolved a preparietal. This future histological work will hopefully be able to test whether these neomorphic ossifications are homologous. The therapsid preparietal can help shed light on the development and evolution of a neomorphic cranial element in the vertebrate fossil record.